Transparent substrate with antiglare coating having abrasion-resistant properties

ABSTRACT

Transparent, especially glass, substrate ( 10 ) having at least one functional element ( 20 ) on one face and an antireflection coating ( 11 ) on the opposite face, characterized in that the substrate includes an abrasion-resistant antiscratch coating, said coating being formed by the antireflection coating ( 11 ) made from a stack of thin dielectric layers having alternating high and low refractive indices.

The invention relates to transparent, most particularly glass,substrates which are provided with an antireflection coating withabrasion resistance properties. It also relates to their use as glazingor filters for display screens, for example plasma screens.

The function of an antireflection coating deposited on a transparentsubstrate is to reduce its light reflection, and therefore to increaseits light transmission. A substrate thus coated therefore sees anincrease in its transmitted light/reflected light ratio, therebyimproving the visibility of objects placed behind it. When it is desiredto achieve the maximum antireflection effect, it is then preferable toprovide both faces of the substrate with this type of coating.

It can then be employed in many applications, for example for protectinga painting illuminated by a light placed behind the observer, or forconstituting or forming part of a shop window so that what is in thewindow is more clearly distinguishable, even when the interior lightingis low compared with the exterior lighting.

The performance of an antireflection coating can be measured orevaluated according to various criteria. Coming first, of course, areoptical criteria. For an antireflection coating to be considered “good”,it must be able to reduce the light reflection of a standard clear glasssubstrate down to a given value, for example 2%, or even 1% and less.

The multilayer coating is often used for glazing in buildings andautomobiles. It comprises thin interferential layers deposited on thesubstrate, generally an alternation of high and low refractive indexlayers.

International patent application WO 01/37006 teaches a multilayerantireflection coating that meets the criterion of good optical qualityand also has the advantage of maintaining this quality within a widerange of angles of incidence oblique to the vertical, possibly rangingfrom 50 to 700 of inclination. This type of coating is particularlyappreciated for windshield-type glazing and shop counters.

Another antireflection coating is known, consisting of a plasticantireflection film which is used, however, for display screens of anytype, and for example plasma screens. Such a film is generally combinedwith a transparent window serving as filter for the display screen.

A plasma screen comprises a plasma gas, trapped between two glasssheets, and phosphors placed on the internal face of the rear sheet ofthe screen. When the screen is operating, the interactions between theplasma gas particles and the phosphors cause the radiation ofelectromagnetic waves that lie within the near infrared between 800 and1000 nm, the propagation of these electromagnetic waves, mainly throughthe front face of the screen, may be the cause of very troublesomeinterference, especially in the case of equipment placed nearby andcontrolled by infrared, for example using remote control means.

Moreover, like all electronic equipment, plasma screens possessaddressing systems (drivers) which may generate radiation that isparasitic with regard to other devices, such as microcomputers, portabletelephones, etc., with which they must not interfere.

To eliminate, or at the very least reduce, the propagation of suchradiation, one solution consists in placing against the front face ofthe stream a window that is both transparent and metallized in order toprovide electromagnetic shielding. Thus, this shielding may comprise oneor more metal layers, especially silver layers, and/or one or more metalgrids. This shielding may advantageously be provided by a stack of atleast two metal layers made of silver which are either depositeddirectly on the glass substrate constituting the window and facing thescreen or are deposited on an adhesive film bonded to the substratefacing the screen. Generally, this adhesive film has an antireflectionfunction (with respect to light radiation coming from the screen) inorder to improve the brightness of the screen by reducing the lightreflection. It also plays a role in protecting the conducting silverlayers from chemical corrosion.

The substrate of the window may be made of toughened glass oruntoughened glass.

Provided on the external face of the substrate, that is to say on theopposite side from the screen and facing the observer, is also anadhesive film which is capable of retaining the glass fragments, shouldthe toughened glass break and fragment, and which furthermore fulfillsan antireflection function for the comfort of the observer. The film isplaced last on the substrate, as it would not be able to withstand thevarious treatments that the substrate may undergo for the purpose ofdepositing functional elements, such as the stack of metal layers.However, it may happen that, despite all the precautions taken, the facethat has to receive this film is scratched by the various substratehandling operations during the treatment steps or even by the treatmentsthemselves, and this is prejudicial both to the quality of the surfaceand the strength of the substrate itself. The substrate will then beunable to be used and will be scrapped.

Furthermore, since the film is placed on the external face of theprotective window intended to be combined with a display screen, therepeated actions of cleaning the protective screen will end up damagingthis antireflection film. This impairs the antireflection properties andcauses a haze to the detriment of vision, the film then having to bereplaced.

Thus, the object of the invention is to provide a substrate, intended inparticular for display screen filters, which has at least one functionalelement on one face and an antireflection coating on its opposite face,while being ensured that the substrate does not have defects of thescratch type on leaving its manufacturing process and that theantireflection coating is durable and abrasion resistant.

According to the invention, the substrate, especially a glass substrate,having at least one functional element of the metal element type on oneface and an antireflection coating on the opposite face, ischaracterized in that the substrate includes an abrasion-resistantantiscratch coating, said coating being formed by the antireflectioncoating made from a stack of thin dielectric layers having alternatinghigh and low refractive indices.

The invention also relates to a transparent, especially glass, substratehaving an antireflection coating on at least one face, characterized inthat the substrate includes an antiscratch coating having a resistanceof at least 3H and with an abrasion resistance such that the haze of thesubstrate that may be caused remains less than 1.5%, said coating beingformed by the antireflection coating made from a stack of thindielectric layers of alternating high and low refractive indices.

According to one feature, the multilayer antireflection coating isdeposited on the substrate before the functional element is deposited.

Thus, the substrate coated firstly with the stack of antireflectionlayers does fulfill its antireflection function as desired, and thisallows the substrate to be treated in order to deposit functionalelements on one of the faces without any risk of damaging the oppositeface, since the latter has already been coated with an antireflectioncoating formed by the stack of antireflection layers.

According to another feature, the stack is based on Si₃N₄ or SnO₂ andSiO₂ layers. It has been found in fact that this type of combination oflayers is particularly abrasion resistant and is so for relatively largeareas, that is to say well in excess of 10 cm².

Preferably, the stack comprises:

-   -   a high-index first layer (c1) having a refractive index n₁        between 1.8 and 2.2 and a geometrical thickness e₁ between 5 and        50 nm;    -   a low-index second layer (c2) having a refractive index n₂        between 1.35 and 1.65 and a geometrical thickness e₂ between 5        and 50 nm;    -   a high-index third layer (c3) having a refractive index n₃        between 1.8 and 2.2 and a geometrical thickness e₃ between 70        and 120 nm; and    -   a low-index fourth layer (c4) having a refractive index n₄        between 1.35 and 1.65 and a geometrical thickness e₄ of at least        80 nm.

The inventors thus had the not obvious idea of using the antireflectionmultilayer stack as abrasion-resistant antiscratch coating forsubstrates that are already large in size, such as glazing or displayscreen filters, which hitherto used plastic films as antireflectioncoating and had no antiscratch properties.

Consequently, the invention also relates to glazing or a filter,characterized in that it is composed of the single substrate providedwith a stack of antireflection layers on one of its faces and with thefunctional element, formed from a metallic element for the purpose ofelectromagnetic shielding, on its other face. The application isespecially intended for the manufacture of filters for display screens,such as plasma screens.

The metallic element consists, for example, of at least one conductingmetal layer, or rather of a stack of thin layers including at least twosilver layers. Advantageously, the conducting metal layer is based onsilver and forms part of a multilayer stack having the followingsequence:

-   -   Si₃N₄/ZnO/Ag/Ti/Si₃N₄/ZnO/Ag/Ti/ZnO/Si₃N₄.

As a variant, the functional element may consist of a network of wiresin the form of a grid, or else of a combination of a stack ofsilver-based thin layers and a network of wires in the form of a grid.

The functional element may be deposited directly on the substrate ordeposited on a plastic film bonded to the substrate, or else it may belaminated between two plastic films, one of which is bonded to thesubstrate whereas the other is bonded to another substrate.

In the case of display screens, the functional element is preferablycombined with an antireflection coating, which may either be anantireflection multi-layer stack or an adhesive antireflection film.

Finally, it will be preferred to use a substrate made of untoughenedglass for the display screen application, since the substrate, equippedwith one or more functional elements and with the stack ofantireflection layers, may thus be manufactured with a large area andeasily cut to the dimensions of display screen filters without any riskof the fully equipped substrate breaking.

Other advantages and features of the invention will become apparent onreading the description that follows, in conjunction with the appendeddrawings in which:

FIG. 1 is a schematic sectional drawing of a filter according to theinvention combined with a plasma screen;

FIG. 2 illustrates a graph comparing various antireflection coatings;

FIG. 3 shows two images of a coating, of the film type and of themultilayer stack type, respectively; and

FIG. 4 shows a sectional view of an alternative filter according to theinvention, combined with a plasma screen.

It should firstly be pointed out that the various dimensions, especiallythicknesses, of the elements of the invention in the drawings are not toscale so that they are easier to examine.

FIG. 1 illustrates a transparent window 1 intended to be joined to thefront face of a plasma screen E. This window constitutes a filterproviding electromagnetic shielding protection and antireflectionprotection.

The transparent window 1 is formed from a single substrate, such as aglass sheet 10, on which is deposited, on the one hand against theinternal face, that facing the screen, a first functional element 20formed by a stack of metal layers having electromagnetic shieldingproperties and a second functional element 21 formed by an internalantireflection coating covering the multilayer stack 20 and, on theother hand against the external face, on the opposite side from thescreen and facing a possible observer, a multilayer stack 11 whichconstitutes not only an antireflection coating but also anabrasion-resistant antiscratch coating.

The substrate 10 is made of a clear soda-lime silicate glass of thePLANILUX type with a thickness of 3 to 6 mm, for example 4 mm. Thisproduct is sold by Saint-Gobain Glass.

To take an example, the stack of metal layers 20 is formed by at leasttwo electrically conducting functional layers, of the Ag type. Thesemetal layers are inserted into a stack of thin protective layers, apreferred sequence of which is as follows:

-   -   glass/Si₃N₄/ZnO/Ag/Ti/Si₃N₄/ZnO/Ag/Ti/ZnO/Si₃N₄.

The Ti layer constitutes a metal layer for protecting the silver,especially by preventing the silver from oxidizing.

A TiO₂ layer may be inserted between the Si₃N₄ and ZnO layers close tothe glass so as to “wash” the color of the substrate in reflection.

All the layers of the stack are deposited by a known sputteringtechnique onto the internal face of the substrate, that intended to facethe screen. The layers are in all cases deposited directly on the glasssubstrate and not on a plastic film, as is sometimes carried out in theprior art, since, as will be seen by the results of Example 3 later, afilm supporting these metal layers generates more haze.

The first metal layer, of Ag, placed closest to the substrate has athickness e₁ approximately equivalent to the thickness e₂ of the secondmetal layer, of Ag, so that the thickness ratio e₁/e₂ is between 0.8 and1.1, and preferably between 0.9 and 1. Thus, the light transmission,greater than 67%, is very suitable. The total thickness e₁+e₂ of themetal layers is between 27 and 30 nm. To obtain good reflection of theinfrared radiation toward the screen, that is to say that the radiationpasses through the substrate as little as possible, it will be preferredto choose a total thickness of the metal layers between 28 and 29.5 nm,the transmission of the radiation thus not exceeding 13% for awavelength of 800 nm.

The table below gives an example of the thickness values for the variousthin layers of the stack, with total thicknesses e₁ plus e₂ equal to 14nm. Glass Thickness (nm) Si₃N₄ 20 TiO₂ 5 ZnO 10 Ag 14 Ti 1.5 Si₃N₄ 73ZnO 10 Ag 14 Ti 1.5 ZnO 10 Si₃N₄ 22.5

Of course, other variations of metal elements for electromagneticshielding are conceivable, such as metal grids or a combination of gridsand layers. Thus, reference may be made to international patentapplication WO 01/81262. Furthermore, the functional element 20providing electromagnetic shielding is deposited directly on the glass,but may as a variant be deposited on an adhesive plastic film bonded tothe internal face of the window, or else may be laminated between twoplastic inserts I adhesively bonded to two substrates, such as thesubstrate 10 and a glass substrate 10 a facing the screen, the substrate10 being provided with the internal coating 21 (FIG. 4).

According to a first embodiment, the internal antireflection coating 21covering the multilayer stack 20 may be a standard plasticantireflection film 21 a or a multilayer stack 21 b of the stack 11type.

The multilayer stack 21 b of the 11 type makes it possible, comparedwith the film, which may be fragile, for the silver layers of the stack20 to be very suitably protected from corrosion because they are coveredby silica or silicon nitride.

However, the advantage of a film for the functional element 21, apartfrom all the glass splinters being retained by it should the windowbreak, is that it may have an effect on the contrast of the screen.There are various types of film, these depending on their lighttransmission. Thus, it will be possible to use a film having a lighttransmission of 80% sold by Calsak or else one having a lowertransmission, to improve the contrast of the screen, such as a film of70% light transmission sold by Southwall (product name ARA2-70).

Finally, the antireflection multilayer stack 11 according to theinvention is a stack of at least two layers based on Si₃N₄ or SnO₂ andSiO₂, which are particularly suitable because of their hardnessproperty. These layers were all deposited conventionally by magneticallyenhanced reactive sputtering in an oxidizing atmosphere using an Sitarget or a metal target to make the SiO₂ or metal oxide layers, usingan Si or metal target in a nitriding atmosphere to make nitrides, and ina mixed, oxidizing/nitriding atmosphere to make the oxynitrides. The Sitargets may contain another metal in a small amount, especially Zr orAl, in particular so as to make them more conducting.

A preferred antireflection stack is the following:

-   -   glass/SnO₂ (or Si₃N₄)/SiO₂/SnO₂ (or Si₃N₄)/SiO₂.

The table below gives the index n_(i) and the geometrical thicknesse_(i) in nanometers of each of the layers, the layer c1 being thatagainst the glass. LAYER (c1) LAYER (c2) LAYER (c3) LAYER (c4) n_(i) 2.01.46 2.0 1.46 e_(i) 15 nm 30 nm 70 nm 90 nm

The purpose of this example is to minimize as far as possible the valueof the light reflection R_(L) of the glass (on the observer's side) at60° incidence.

The table below gives results in light reflection on the observer's sideand of optical haze of the filter-screen product according to theexample (example 1) of the invention and according to two comparativeconfiguration examples (examples 2 and 3). For the comparative examples,the filters both had, as functional element 21 and antireflectioncoating 11, antireflection films on either side of the substrate, butthe stack 20 of Ag layers is, in the case of one of them, depositeddirectly on the glass (example 2) and in the case of the other onedeposited on a plastic film (example 3):

-   -   Example 1: adhesive antireflection film 21 a/stack of Ag        layers/glass/multilayer stack 11;    -   Example 2: adhesive antireflection film/stack of Ag        layers/glass/adhesive antireflection film;

Example 3: adhesive antireflection film/stack of Ag layers on plasticfilm/glass/adhesive antireflection film. Example 1 Example 2 Example 3Light reflection, 3.3 5.3 2.7 observer's side (%) Optical haze, 0.3 0.61.3 observer's side

Example 1 of the invention is clearly the one to adopt, as themultilayer antireflection coating 11 has the advantage of reducing thelight reflection on the external face of the window, on the observer'sside, while producing a less hazy product after manufacture. Thereduction in haze (scattered light) improves the quality of the imageand the observer's viewing quality—in particular, the colors arestronger.

According to the invention, the antireflection multilayer stack alsoconstitutes an abrasion-resistant antiscratch coating. Given below is acomparative table and, with regard to FIG. 2, a comparative graph,comparing the example of the invention (example 1) with example 2,relating to the durability test, or accelerated aging test, and to theabrasion test, respectively.

As regards the durability test, the change in light transmission T_(L)and in haze were measured after the window spent 500 hours in dry heatand 120 hours in wet heat. 90% RH; 500 h at 80° C. 60° C.; 120 h ΔT_(L)Δ haze ΔT_(L) Δ haze (%) (%) (%) (%) Example 1 <0.2 <0.2 <0.3 <0.2Example 2 <0.5 <0.4 <1 <0.2

The two products behave identically in the accelerated aging test.

As regards the abrasion test, the change in the haze after abrasion wasmeasured the well-known way using grinding wheels of the Taber Abraser®CS 10 F type with a load of 250 grams on a window of example 1 and on awindow of example 2.

The graph in FIG. 2 shows that the window of the invention exhibits muchbetter abrasion behavior in the Taber test than the window of the priorart. After 500 revolutions, the coating with the multilayer stack ishardly scratched, whereas the coating with the plastic film iscompletely torn, resulting in a complete loss of antireflectionproperties and causing substantial haze. The micrographs in FIG. 3, witha magnification of 80 times, show the appearance of the two productsafter the test; the window with the multilayer stack is much cleaner.

Furthermore, the measurements made regarding the scratch resistanceshows that the resistance with a multilayer stack is much greater thanthat with the film. It is only 2 to 3H in the case of the film, whereasit is more than 3H in the case of the multilayer stack. It will berecalled that H is the classification used in the field of surfacecoatings and organic substrates; it involves a scratch test using a 2Hor 3H pencil lead.

The window combined with a plasma screen was taken as an example, butthe invention applies, of course, to any type of substrate that possiblyhas to have at least one functional element on one face and anabrasion-resistant antiscratch coating on its opposite face, whichcoating may optionally have the advantage of being antireflecting.

1. A transparent, substrate (10) having at least one functional element(20) on one face and an antireflection coating (11) on the oppositeface, said coating being made from a stack of thin dielectric layershaving alternating high and low refractive indices, characterized inthat the antireflection coating is used as abrasion-resistantantiscratch coating.
 2. The transparent substrate as claimed in claim 1,characterized in that the abrasion-resistant antiscratch coating formedby the antireflection coating (11) has a resistance of at least 3H andwith an abrasion resistance such that the haze of the substrate that maybe caused remains less than 1.5%.
 3. The substrate as claimed in claim1, characterized in that the multilayer antireflection coating isdeposited on the substrate before the functional element is deposited.4. The substrate as claimed in claim 1, characterized in that themultilayer stack is based on Si₃N₄ or SnO₂, and SiO₂.
 5. The substrateas claimed in claim 1, characterized in that the stack comprises, insuccession: a high-index first layer (c1) having a refractive index n₁between 1.8 and 2.2 and a geometrical thickness e₁ between 5 and 50 nm;a low-index second layer (c2) having a refractive index n₂ between 1.35and 1.65 and a geometrical thickness e₂ between 5 and 50 nm; ahigh-index third layer (c3) having a refractive index n₃ between 1.8 and2.2 and a geometrical thickness e₃ between 70 and 120 nm; and alow-index fourth layer (c4) having a refractive index n₄ between 1.35and 1.65 and a geometrical thickness e₄ of at least 80 nm.
 6. Thesubstrate as claimed in claim 5, characterized in that the stack is asfollows: Si₃N₄/SiO₂/Si₃N₄/SiO2.
 7. The substrate as claimed in claim 1,characterized in that the functional element (20) is a metallicelectromagnetic shielding element.
 8. The substrate as claimed in claim7, characterized in that the functional element (20) consists of atleast one conducting metal layer.
 9. The substrate as claimed in claim7, characterized in that the functional element (20) consists of a stackof thin layers including at least two silver layers.
 10. The substrateas claimed in claim 8, characterized in that the multilayer stack hasthe following sequence: Si₃N₄/ZnO/Ag/Ti/Si₃N₄/ZnO/Ag/Ti/ZnO/Si₃N₄. 11.The substrate as claimed in claim 7, characterized in that thefunctional element (20) consists of a network of wires in the form of agrid.
 12. The substrate as claimed in claim 7, characterized in that thefunctional element (20) consists of the combination of a stack ofsilver-based thin layers and a network of wires in the form of a grid.13. The substrate as claimed in claim 7, characterized in that thefunctional element (20) is deposited directly on the substrate (10). 14.The substrate as claimed in claim 7, characterized in that thefunctional element (20) is deposited on a plastic film bonded to thesubstrate (10).
 15. The substrate as claimed in claim 7, characterizedin that the functional element (20) is laminated between two plasticfilms, one of which is bonded to the substrate (10) whereas the other isbonded to another substrate (10 a).
 16. The substrate as claimed inclaim 7, characterized in that the functional element (20) is combinedwith a second functional element (21) made of an antireflection coating.17. The substrate as claimed in claim 16, characterized in that thesecond functional element (21) is an antireflection multilayer stack (21b).
 18. The substrate as claimed in claim 16, characterized in that thesecond functional element (21) is an adhesive antireflection film (21a).
 19. The substrate as claimed in claim 1, characterized in that thesubstrate is made of untoughened glass.
 20. The application of thesubstrate as claimed in claim 1, to the manufacture of glazing or offilters for display screens.
 21. The application as claimed in claim 20for plasma screens.